Tunneling barriers in electrochemical scanning tunneling microscopy

We have measured barriers for electron tunneling between a Pt-Ir tip and a gold substrate under potential control, obtaining values similar to those reported in ultrahigh vacuum. However, in contrast to vacuum tunneling, the data show a strong dependence on the bias applied between the tip and the substrate. They are only weakly dependent on the electrochemical potential of the substrate. The barrier changes with the direction of electron tunneling, an effect we attribute to permanent polarization in the gap. We observe a sharp dip near zero bias for tunneling in water. It is not observed for tunneling in a nonpolar solvent, and we attribute it to induced polarization in the tunnel gap.